The present invention generally relates to photoelectric conversion devices and more particularly to a photoelectric conversion device used in the scanners, and the like.
Conventionally, photosensors of the photoconduction type are known wherein amorphous silicon is used as the photoconductive cell. In this type of photosensor, the conductivity of amorphous silicon is changed in response to the optical radiation, and the detection of the optical radiation is made by detecting the change in the electric current flowing through the amorphous silicon. Such a photosensor is characterized by the excellent response that is far superior to the photosensors that use chalcogenides as the photoconductive cell. Thus, the photosensor of the foregoing type is suitable particularly for high speed scanners for reading images at high speed.
In spite of the foregoing various advantages, the photosensor based on amorphous silicon has a problem in that the sensitivity is relatively low due to the small photoconductive current flowing through the amorphous silicon photoconductive cell. In other words, the conventional photosensor of the type described suffers from a problem in that the photosensor easily picks up noise. Further, there is a tendency that the sensitivity is changed device by device. Such a change in the sensitivity should be eliminated, as the change may cause unstable detection of images.
Because of the low sensitivity of the photosensor, one has to use a low noise circuit for the detection circuit to drive the photosensor. However, such a low noise circuit is generally expensive, and increases the cost of the scanner. Further, a large area has been needed in the conventional photosensor for receiving the optical radiation so that the signal-to-noise ratio is increased as much as possible. However, such a large area obviously decreases the integration density when the photosensor is realized as an integrated circuit.
FIG. 1 shows an example of the conventional photoelectric conversion device.
Referring to FIG. 1, the device includes a voltage source 21 for supplying a predetermined voltage, a photoconductive cell 22 of amorphous silicon connected to the voltage source 21 for obtaining electric current therefrom, a load capacitance 23 connected in series to the photoconductive cell 22, and an amplifier 24 connected at a node between the photoconductive cell 22 and the load capacitance 2 for detecting the electric voltage at the node.
When the photoconductive cell of amorphous silicon is used, one can usually obtain a photoconductive current in response to the optical radiation. For example, in response to the optical radiation having the intensity of about 100 1x, a photoconductive current larger by a factor of 10.sup.2 than the dark current is obtained. However, this ratio between the photoconductive current and the dark current inevitably decreases with increasing integration density of the photosensor because of the reduced area for receiving the optical radiation. Such a decrease in the photoconductive current is the reason why the capacitance 23 is used as the load rather than the load resistance. Further, because of the extremely small magnitude of the current, one has to use an extremely stable voltage source 21 such that there is substantially no noise included in the output voltage applied to the photoconductive cell 22. However, capacitance is not easy to fabricate in integrated circuits as compared to resistors, and thus, the use of the load capacitance 23 causes an increase in the cost of the photosensor. Similarly, the necessity of the stabilized voltage source 21 also contributes to the increased cost of the photosensor.